This invention relates to fiber-optic communications systems, and more particularly, to dynamic gain adjusting in optical communications systems.
In optical networks that use wavelength division multiplexing, multiple wavelengths of light are used to support multiple communications channels on a single fiber. Optical amplifiers are used in such networks to amplify optical signals that have been subject to attenuation over multi-kilometer fiber-optic links.
There are many fiber spans in a typical network link. For example, a long-haul network link may be approximately 400-600 km in length and an ultra-long-haul network link may be 3000-5000 km in length. Each fiber span is typically 40-130 km in length, so there may be many amplifiers in such links.
A typical amplifier may include erbium-doped fiber amplifier components that are pumped with diode lasers. Erbium-doped fiber amplifier stages increase the strength of the optical signals being transmitted over the fiber-optic links.
The intrinsic gain spectrum of erbium-doped fiber amplifiers is not flat. Unless the gain spectrum of an erbium-doped fiber amplifier is flattened, different wavelengths of light will be amplified by different amounts. This is undesirable, particularly in arrangements in which many amplifiers are cascaded in a communications link.
One way in which to flatten the gain spectrum in an erbium-doped fiber amplifier is to use a gain equalization filter. A gain equalization filter may be inserted in the optical path of the erbium-doped fiber amplifier between coils of erbium-doped fiber. The gain equalization filter has a loss spectrum that tracks the erbium-fiber gain spectrum. The gain equalization filter may therefore be used to introduce losses in the portions of the spectrum where the erbium-doped fiber has gain peaks. This flattens the gain spectrum of the amplifier.
Two important figures of merit for an optical amplifier are gain ripple and noise figure. Gain ripple is a measure of the spectral variations in the gain of the amplifier. The noise figure for an amplifier is an indicator of the amount of noise that the amplifier adds to the optical signal that is being amplified.
Gain ripple may arise from the use of imperfect gain equalization filters. Gain ripple may also arise from the use of other amplifier components with wavelength-dependent losses. For example, wavelength-division multiplexer (WDM) couplers and other components may introduce wavelength-dependent losses that affect gain ripple. Dynamic contributions to gain ripple include spectral hole burning and stimulated Raman scattering. With spectral hole burning, the gain of the erbium fiber is reduced at wavelengths immediately adjacent to an active signal channel. Raman scattering may influence gain ripple when the optical signals being amplified by the amplifier are sufficiently strong that the signals on some channels produce Raman gain for signals on other channels. There may also be drift in the gain spectrum of an amplifier due to fiber and component aging effects.
Gain tilt is a type of gain ripple that may arise when the input power to the amplifier changes. Gain tilt has previously been controlled using variable optical attenuators. Such attenuators may, however, adversely affect the noise figure of the amplifier when large values of attenuation are used to correct for the gain tilt introduced when handling relatively large input powers.
The performance of optical amplifiers in optical communications systems is generally monitored using optical spectrum analyzers. An optical spectrum analyzer in a communications system installation may be used to monitor the performance of a number of optical amplifiers.
It is an object of the present invention to provide an optical device and method for adjustably and dynamically controlling the spectral gain of an optical signal. It is a further object of the present invention to provide an optical device and method for compensating for gain ripple, gain tilt, and/or other gain variations, whatever the source. It is further object of the present invention to provide an optical device and method in which the optical characteristics of an optical amplifier may be monitored and controlled to reduce amplifier noise and gain ripple, gain tilt, and/or other gain variations.
This and other objects of the invention are accomplished in accordance with the principles of the present invention by providing an optical device having a dynamic spectral filter or dynamic spectral gain adjusting device, such as a dynamic gain flattening filter, with dynamically-adjustable spectral gain characteristics, the dynamic gain adjusting filter including at least one semiconductor optical amplifier. The dynamic gain adjusting device, such as a dynamic gain flattening filter, may be used to reduce gain ripple, control gain tilt, and/or compensate for other spectral gain variations.
A dynamic gain adjusting filter in accordance with the present invention may be part of an amplifier module or other optical device module, or as a module itself. Further, the dynamic gain adjusting filter may be implemented with other optical components, including, for example, a fixed gain equalization filter that adjusts the optical signal spectrum through attenuation.
The dynamic gain adjusting filter may be designed and operated to produce a desired spectral gain or loss profile for optical signals passing through it. The dynamic gain adjusting filter includes at least one semiconductor optical amplifier. For example, dynamic gain adjusting filter may be implemented by a series connection of multiple semiconductor optical amplifiers.
One or more semiconductor optical amplifiers of the dynamic gain adjusting filter may be designed and/or controlled to produce different gain profiles. For example, the active region of the semiconductor optical device may comprise several subregions having different structures, dimensions, and/or doping characteristics. Processes for forming the subregions are described. Moreover, the composite spectral gain profile may be adjustable by varying the temperature and/or the bias voltage across the active region or one or more of the subregions.
The optical signals may pass through an active region of a semiconductor optical amplifier from one end to another. One end of the semiconductor optical amplifier may include a mirrored or partially mirrored surface so that optical signals will be reflected, at least in part, by the surface and travel back through the active region. Optical devices and packaging may be provided to facilitate either of these arrangements.